Chemically highly resistant material

ABSTRACT

Thin films of boron nitride become more resistant to environmental influences and exhibit a better etching behaviour (a Permalloy etch instead of hydrofluoric acid can be used) when titanium (zirconium, hafnium) is provided together with boron nitride during the provision of the films, for example, by cathode sputtering. Dependent on the titanium content the thin films constitute, for example, insulators or resistors. The BN/Ti films have eminent sliding properties and may be used, for example, as a solid lubricant and for potentiometers.

United States Patent 1191 Dimigen et al.

[ Oct. 28, 1975 1 1 CHEMICALLY 'HIGHLY RESISTANT 3,242,006 3/1966 Gerstenberg 204/192 MATERIAL 3,256,103 6/1966 Roche et a1... 252/520 X 3,275,915 9/1966 Harinxma 317/258 1 Inventors: Heinz g Hubertus Hiibsch; 3,437,592 4/1969 13665 et a1. 252/12 l-lolger Liithje, all of Hamburg, 3,551,247 12/1970 Feakes 161/182 Germany 3,575,858 4/1971 Adair et a1. 1 252/25 3,730,507 5/1973 Montgomery..... 117/123 B [73] AS51211:Z Ph'hPs Corporal", New 3,824,113 7/1974 Loxley et a1. 117 114 R York, 3,841,848 10 1974 Kasai et al.: 161/213 x [22] Filed: July 16, 1973 Primary Examinerl-laro1d Ansher [21] Appl N05 379,459 Attorney, Agent, or Firm-Frank R. Trifari; Leon Nigohosian [30] Foreign Application Priority Data July 22, 1972 Germany 2236074 ABSTRACT Thin films of boron nitride become more resistant to 1 1 428/472; 204/192; environmental influences and exhibit a better etching 252/26; 252/512; 252/520; 428/913 behaviour (a Permalloy etch instead of hydrofluoric [5 Int. Ciacid can be used) when titanium irconium hafnium) CIOM 7/ O2 is provided together with boron nitride during the pro- Field of Search 1 114 vision of the films, for example, by cathode sputtering.

1 17/160 Dependent on the titanium content the thin films con- 26 stitute, for example, insulators or resistors. The BN/Ti films have eminent sliding properties and may be used, 1 References Clted for example, as a solid lubricant and for potentiom- UNlTED STATES PATENTS eters.

2,156,803 5/1939 Cooper et a1. 252/25 8 C i 6 Drawing Figures 2,842,837 7/1958 Huet et a1... 3,003,885 10/1961 Mandorf 252/520 X 0 I I I T I US. Patent Oct. 28, 1975 Sheet10f4 3,916,075

US. Patent 0a. 28, 1975 Sheet 2 of4 3,916,075

US. Patent 0a. 28, 1975 Sheet 3 of4 3,916,075

US. Patent Oct. 28, 1975 Sheet4 of4 3,916,075

10 3 2. 567910 ''i'sno Fig.5

CHEMICALLY HIGHLY RESISTANT MATERIAL The invention relates to chemically high resistant material which is provided in the form of thin films on substrates.

In this respect thin films are understood to mean films having a thickness of from 100 A to 10 ,um particularly having a thickness of from 1 to 2 pm. The significance of the term chemically highly resistant will be come apparent hereinafter. In accordance with the various technologies described hereinafter a great variety of materials such as, for example, glass, metal, synthetic material, quartz and silicon discs are suitable as a substrate.

In a technology of integrated circuits stringent requirements are imposed on the materials of passive elements (for example, insulators, capacitors and resistors) used for the construction. The extent to which these requirements are satisfied does not only depend on the properties which are determined by the function of the passive elements but also on the requirements which are imposed on the active elements (transistors) which, as is known, are also present in integrated circuits. A special requirement is the satisfactory adhesion to the substrate as well as a satisfactory selective etch ing behaviour of all materials present in an integrated circuit.

For example, insulators which consist of, for example, SiO SiO or A1 or capacitors which comprise, for example, thin films of tantalum oxide or titanium dioxide must not only have the required electrical properties (low dielectric constant, small loss factor, high breakdown strength) but also a high chemical resistance and a satisfactory selective etching behaviour in order that a satisfactory structuring of all film materials present in the relevant construction is possible.

The materials for passive elements used in the technology of the integrated circuits only partly satisfy these conditions so that chemical etching processes cannot be used in many cases for structuring the films.

Also when manufacturing optical elements thin films having a defined refractive index are increasingly being used. The field of use ranges from miniaturized optical elements in the integrated technique (for example, electro optical converters) to coating of optical elements (for example, lenses). In some of these fields of use there is a need of materials having a high chemical resistance, for example, a resistance to hydrofluoric acid.

Furthermore the significance of solid lubricants in the .form of thin films is increasing. For example, the use of sputtered M05 films, i.e. films provided by cath ode sputtering, as a lubricant is known (ASLE Proceedings 1971, issued by the American Society of Lu brication Engineers). However, these films have the drawback that they are sensitive to chemically active materials.

Consequently the term chemically highly resistant is not only understood to mean a high resistance to environmental influences, for example, tapwater and industrial air but also a selective etching behaviour relative to the chemicals used in the above-mentioned technologies, for example, hydrofluoric acid.

It is known that pure and compact boron nitride has eminent dielectric properties, is resistant to most corrosives and to oxidation even at high temperatures and is not moistened by many molten materials. Therefore boron nitride is a very suitable material particularly when it can be manufactured as a thin film tightly adhering to the surface of the article, which film is uniform, evenly compact and resilient. For example, boron nitride films may be advantageously used as dielectrics namely as insulators for leads, as fuses or as dielectrics in capacitors (German Auslegeschrift l ,52 l ,5 28

During the investigations which led to the present invention it was, however, found that the resistance of thin films of boron nitride to the above-mentioned environmental influences leaves much to be desired. Furthermore the selective etching behaviour which is required for the above-mentioned technologies is not satisfactory.

It is therefore an object of the present invention to provide highly resistant material which also has a selective etching behaviour which is optimum for the various above-mentioned technologies.

The invention is characterized in that the material constituting the thin film is boron nitride which comprises 0.5 to by weight of at least one element from the IVth sub-groupof the Periodic Table of Elements. The elements from the IVth sub-group of the Periodic Table are titanium, zirconium and hafnium. As is known, these materials are very similar as regards their behaviour so that they may replace each other entirely or partly within the scope of the invention. Consequently if for the sake of simplicity the invention is described in detail with reference to titanium only, this should not be considered as a limitation of the invention because corresponding examples can be given with zirconium and hafnium.

For manufacturing the material according to the invention the following methods may be used:

Vapour deposition processes in which the material to be provided as a thin film is heated in a suitable ambiance (for example, in vacuo or in a shielding gas) to such an extent that it evaporates and is deposited as a film on the substrates.

Plasma spray processes in which the material to be provided as a thin film is brought into a plasma are as a fine-grained powder. In that case the powder particles melt and are deposited as a film on adjacently provided substrates.

Cathode sputtering, also referred to as RF ionic sputtering in which the material to be provided as a thin film is brought as a target in a gas discharge in such a manner that the material is sputtered by ion bombardment. The removed (sputtered) particles are deposited as a thin film on the substrate which is provided adjacent in the counter electrode (substrate holder). Due to the choice of the filler gas in which the ionsare carriers for the occurring independent gas discharge, the composition of the film provided by sputtering may be influenced.

Although all these and analogous methods may be used within the scope of the present invention, the invention will hereinafter be described in greater detail by way of a method using cathode sputtering.

The invention therefore also relates to a method of providing thin films of a chemically highly resistant material on substrates and is characterized in that at least one target of boron nitride and of an element from the IVth sub-group of the Periodic Table is subjected in the presence of a gas to a cathode sputtering treatment.

It is possible to use, for example, one target which comprises the desired quantities of the elements from the IVth sub-group of the Periodic Table; it is alternatively possible to use, for example, two targets one of which consists of boron nitride and the other consists of, for example, titanium. A further possibility is to subject a boron nitride disc coated with discs of at least one element from the IVth sub-group of the Periodic Table in the presence of a gas to a cathode sputtering treatment.

The properties of the films obtained are dependent on the extent of coating of the boron nitride disc with the smaller discs of, for example, titanium. This provides the possibility of varying these properties. A further possibility of varying these properties is the choice of the gases (nitrogen or argon).

Dependent on the relative quantities of elements from the IVth sub-group of the Periodic Table and on the gas used the material according to the invention is particularly suitable for separate given uses from the group of the various above-mentioned uses.

The properties and advantages of the materials according to the invention will hereinafter be described with reference to the accompanying drawing. In this drawing the graph of FIG. 1 shows the dependence of the etching rates A (in pm per minute) (removal of the film per unit of time) on the titanium content Ti (in by weight) of the film to be preferably used as an insulating material; line 1 for so called Permalloy etchant (see below) and line 2 for so-called titanium etchant (see also below).

FIG. 2 shows in a graph the dependence of the specific resistance p (in Ohm.cm) of the titanium content (Ti in by weight) of the films to be preferably used as a resistive material. (Line 1 for films provided in using nitrogen and line 2 for films provided in using argon as a shielding gas).

FIGS. 3a and 3b show in graphs the etching behaviour of film to be preferably used as a resistive material. A which in these figures is expressed in units of am/min, means the etching rate in both the figures and Ti the titanium content in by weight of the films. Line 1 is for so-called Permalloy etchant and line 2 for the use of so-called titanium etchant, in each figure respectively.

The diagram of FIG. 4 shows the relationship between the refractive index n and the titanium content (Ti, in by weight) of films to be preferably used as an optical material.

The graph of FIG. 5 shows the dependence of the temperature coefficient a (l/C) of the titanium content Ti (in by weight) of films to be preferably used as a resistive material. Line 1 for films provided in argon and line 2 for films provided in nitrogen as a shielding gas.

Thin films of boron nitride and titanium with titanium contents of between 1 and 6% provided in nitrogen by sputtering constitute insulating materials having a high chemical resistance. Such an insulator has the specific resistance The dielectric properties correspond to those of the known SiO insulators. As regards the abovementioned properties, boron nitride-titanium insulators are equivalent to the known insulators. In addition boron nitride-titanium insulators are, however, very resistant to hydrofluoric acid. In their etching behaviour the boron nitride-titanium insulators have the advantage that they can already be structured in a Permalloy etchant while SiO alone can be etched in the aggressive hydrofluoric acid. A Permalloy etchant has, for example, the following composition:

100 ml H 0 34 ml H SO, (95%) 18 ml H 0 (30%) Such an etchant can of course be used in a much simpler method than an etchant comprising hydrofluoric acid.

The graph in FIG. 1 shows the etching rates A (in ,um/min.) of boron nitride-titanium insulators comprising 1 to 6% of titanium for the said Permalloy etchant (line 1) and for a so-called titanium etchant (line 2) comprising hydrofluoric acid and having the following composition:

100 ml H 0 5 ml H SO (95%) 1 ml HF (40%) Boron nitride-titanium films comprising 1 to 6% of titanium may be manufactured, for example, by cathode sputtering of a target of boron nitride-titanium material having a titanium content of between 1.5% to 8.5% in a nitrogen atmosphere.

When the ratio between boron nitride and titanium is shifted to a higher titanium content, while nitrogen or argon may be used as a gas, a resistive material is obtained in which an (experimentally determined) correlation is present between titanium content and specific resistance (FIG. 2). A defined specific resistance can be obtained with this material comprising from 5% to 95% of titanium which has as compared with the known resistive materials a better thermal conductivlty.

The wet chemical etching behaviour of this material is found from FIG. 3a (N discharge) and FIG. 3b (Ar discharge). In FIG. 3a a negligibly small etching rate and in FIG. 3b an etching rate (removal of the film per unit of time) which is sufficient for film structures is obtained in the given etching solution. Films which are provided by sputtering both in nitrogen and in argon are resistant to hydrofluoric acid.

Since this material can be provided by sputtering in a nitrogen or argon discharge, the titanium content of the target in case of an N discharge must be between 8.5% and 97% and in case of an Ar discharge must be between 2% and 90% for the envisaged range of compositions of the film (5% to 95% of Ti).

In addition to the above-mentioned properties the films thus manufactured exhibit a satisfactory thermal conductivity and an eminent sliding capacity. For many possible uses (for example, in potentiometers) especially the sliding property is of importance.

It is very favourable that the temperature coefficient a of BN/Ti sputtered in Ar in the range having a titanium content of the film of from 40% to 100% is not larger than 3. l0 l/C and in addition has a zero crossing at approximately of titanium (FIG. 5).

Thin films of a boron nitride-titanium material whose titanium content is between 0.5% and IS% constitute a resistive optic material having a variable refractive index.

This material whosc etching behaviour is shown in FIG. 1 can only be etched in the given solutions. As compared with other acids (including hydrofluoric acid) it is found to be resistant.

The refractive index is shown in FIG. 4 as a function of the titanium content.

Films having the optical property as shown in FIG. 4 are provided by sputtering in a nitrogen discharge while a boron nitride-titanium material is used as a target whose titanium content may vary between 0.7% and 21% so as to obtain the desired refractive index. In view of the use of nitrogen as a gas it is probable that the components of this material according to the invention are boron nitride and titanium nitride.

Likewise as M05 boron nitride is among the known solid lubricants. Whereas pure BN films are sensitive to chemical reagents the etching behaviour is modified to a considerable extent already in low doped materials as is apparent from the above-mentioned examples.

This property causes the material according to the invention to be suitable as a lubricant even under circumstances promoting corrosion.

The manufacturing method according to the invention will be described in greater detail with reference to the following embodiment.

A boron nitride disc evenly coated with titanium discs (diameter 2 mm) on its surface was used as a target. The substrates consisted of silicon wafers either coated or not coated with an SiO film and having a thickness of one-fourth mm and a diameter of 33 mm, and glass sheets of the same dimensions. They were present in a substrate holder at a distance of 30 mm from the target so that one of their surfaces faced the target.

The gas discharge was effected at a pressure of l.5.l0 Torr. A transmitter having an operating frequency of 27.12 MHz and a maximum power of 2.5 kW was used as a high-frequency source.

In most experiments the energy density of the ions on the target was approximately 3 W sq.cm. The substrates were not separately heated, but reached a temperature of approximately 300C during sputtering. The time to provide a l ,um thick film varied with the titanium content of the target and was one hour at an average, that is to say, a film of approximately 170 A thick was provided per minute.

What is claimed is:

1. A chemically resistant material which is provided in the form of a thin film on substrates, said material being a mixture consisting essentially of from 0.5 to by weight of at least one element from the group consisting of titanium, zirconium, and hafnium and the balance boron nitride.

2. Use of the material recited in claim 1 as a solid lubricant.

3. A material as in claim 1, containing 1 to 6% by weight of said at least one element.

4. Use of the material recited in claim 3 as an electrically insulating material.

5. A material as in claim 1, containing 0.5 to I57: by weight of said at least one element.

6. Use of the material as recited in claim 5 as an optical material.

7. A material as in claim 1, containing 5 to 95% by weight of said at least one element.

8. A material as in claim 1, wherein said element is 

1. A CHEMICALLY RESISTANT MATERIAL WHICH IS PROVIDED IN THE FORM OF A THIN FILM ON SUBSTRATES, SAID MATERIAL BEING A MIXTURE CONSISTING ESSENTIALLY OF FROM 0.5 TO 95% BY WEIGHT OF AT LEAST ONE ELEMENT FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, AND HAFNIUM AND THE BALANCE BORON NITRIDE.
 2. Use of the material recited in claim 1 as a solid lubricant.
 3. A material as in claim 1, containing 1 to 6% by weight of said at least one element.
 4. Use of the material recited in claim 3 as an electrically insulating material.
 5. A material as in claim 1, containing 0.5 to 15% by weight of said at least one element.
 6. Use of the material as recited in claim 5 as an optical material.
 7. A material as in claim 1, containing 5 to 95% by weight of said at least one element.
 8. A material as in claim 1, wherein said element is titanium. 